CHICAGO – The three police officers charged with conspiring to cover up a fellow officer's actions in the police shooting death of 17-year-old Laquan McDonald failed their oath to protect all city residents and focused instead on protecting a single colleague, prosecutors argued Tuesday.

The trial of former Detective David March, former Officer Joseph Walsh and Officer Thomas Gaffney opened Tuesday less than two months after former Officer Jason Van Dyke was found guilty of second-degree murder and aggravated battery in the 2014 shooting death of the black teen.

March, 60, Walsh, 49, and Gaffney, 45, face state charges of conspiracy, obstruction of justice and misconduct. Prosecutors say they tried to prevent independent investigators from learning the truth about McDonald’s death.

The officers have opted for a bench trial. The case against them will be heard and decided by Cook County Associate Judge Domenica Stephenson, not a jury.

March, Walsh and Gaffney are among several officers who allegedly reported that McDonald acted aggressively toward police and posed an immediate threat.

A police dashcam video of the incident contradicts those claims. It shows McDonald turning away from officers when Van Dyke began shooting.

"This case is about violating the public trust," special prosecutor Patricia Brown Holmes said in her opening statement Tuesday. "It boils down to what the defendants wrote on paper” compared to what the video shows.

The video shows Van Dyke opened fire within six seconds of exiting his police vehicle. McDonald was on the ground within 1.6 seconds. Van Dyke continued to shoot at McDonald for another 12.5 seconds.

He fired 16 shots at the teen.

The video, which played a crucial part in Van Dyke’s trial, will be central in the cover-up trial as well.

James McKay, March's defense attorney, called his client an "honest, decent, professional detective."

In his opening statement, McKay said the charges against the officers have been driven by politics. He said the officers did their best in a difficult situation.

"The officers were not violating the public trust," McKay said. "They were protecting the public."

Officers responding to reports of a person breaking into vehicles in a truck lot on Chicago's Southwest Side on Oct. 20, 2014, found McDonald wielding a knife with a 3-inch blade. Officers would follow the teen for nearly a quarter mile as he wound his way through city streets.

The troubled teen ignored repeated calls from police to drop the knife. He popped the tire of a police vehicle and scratched a windshield.

Van Dyke told investigators that McDonald raised the knife in a menacing manner before he fired, and that he backpedaled as the teen approached. The police video did not support Van Dyke's account.

Walsh said he, too, "backed up" as McDonald got to within 12 to 15 feet of the officers, according to police documents. He said the teen "swung the knife toward the officers in an aggressive manner."

Holmes, the special prosecutor, noted that the video shows Walsh "did not back up but instead advanced towards McDonald."

Walsh, who was Van Dyke's partner on the night of the shooting, is accused of lying to investigators about what led to the shooting. He testified under a grant of limited immunity at Van Dyke’s trial. Walsh's testimony in that trial may not be held against him at his own trial, as long as it was truthful.

March, the lead detective, allegedly signed off on statements given by officers at the scene and indicated that there was no discrepancy between what the officers said happened and police dashcam video.

Gaffney has been suspended without pay since the indictment against the three officers was announced in June 2017. March and Walsh have resigned from the department.

One witness expected to testify in their trial is Officer Dora Fontaine. Fontaine's initial statement to investigators seemed to have been contradicted by the police video.

She told investigators that she heard officers repeatedly order McDonald to drop the knife, according to police documents released by the city. She said McDonald "raised his right arm toward Officer Van Dyke, as if attacking Van Dyke."

Fontaine has been given immunity by prosecutors. She testified during Van Dyke’s trial that she saw McDonald making only "swaying" movements with the knife before the shooting.

McKay questioned Fontaine's credibility. A city inspector recommended last year that department officials fire her. She has been placed on desk duty.

"She has to be a witness in this case, because she needs this job," McKay said. "She did change her story to save her ass.”

The shooting was one in a series of deadly confrontations between law enforcement and black men and women that spurred a national conversation on policing. The way city and police officials handled the incident further fractured the already tattered trust of police in Chicago’s African-American community.

Local leaders initially resisted releasing he video. Activists accused Mayor Rahm Emanuel and police brass of allowing an unofficial “code of silence” among officers to persist.

A court eventually ordered the city to release the footage, 400 days after the shooting. On the day of its release, Van Dyke was charged with first-degree murder.

The county’s prosecutor, Anita Alvarez, accused by activists of taking too long to charge the officer, was voted out of office. And local and federal authorities launched investigations of the police department.

Emanuel saw his standing in the city's sizable African-American community plummet. He fired his police superintendent and announced in September, days before jury selection began in the Van Dyke trial, that he wouldn’t seek a third term in office.